Noise suppressing a c phase control system

ABSTRACT

An a-c phase control circuit includes a power transistor for phase control replacing the thyristor generally used for this purpose.

United States Patent Brungsberg NOISE SUPPRESSING A C PHASE CONTROL SYSTEM Heinrich-Josef Brungsberg, Ludenscheid, Germany Brown, Boveri 8: Cle A.G., Mannheim, Germany Filed: Apr. 20, 1970 Appl. No.: 30,135

Inventor:

Assignee:

Foreign Application Priority Data Apr. 25, 1969 Germany ..P 19 21 122.4

U.S. Cl. ..'.307/257, 307/247, 307/252 W, 307/262, 315/DIG. 4, 323/22 T, 323/34 Int. Cl. ..H03k 17/00, G051 1/44 Field of Search ..307/247, 252 W, 253, 254, 257, 307/262; 323/22 T, 34, 7 SE; 315/DIG. 4

[4 1 June 6, 1972 References Cited Primary Examiner-John Zazworsky Attorney-Erwin Salzer ABSTRACT An a-c phase control circuit includes a power transistor for phase control replacing the thyristor generally used for thispurpose.

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SHEET 3 0F 4 5 INVEN'TORI b M NWW hi Attorney 1 NOISE SUPPRESSING A C PHASE CONTROL SYSTEM BACKGROUND OF INVENTION The tenn a-c phase control is applied in reference to a process of rapid On-OFF switching which connects an a-c supply to a load for a controlled fraction of each cycle. A-C phase control is applied extensively as a means for controlling the average power to various loads such as, for instance, electric lamps, electric heaters and electric motors. Electric circuitry for controlling the average power to an electric lamp by means of a-c phase control is referred to as a phase control dimmer. This invention lends itself well to a-c phase control dimmers.

Generally a-c phase control is achieved by thyristor circuitry including one or more power thyristors. In any circuit including a power thyristor the rate of rise of the current following triggering of the power thyristor is very high. The frequency spectrum of the steeply rising currents encompasses frequencies ranging from about 150 k Hertz to 30 m Hertz. These frequencies constitute radio noise.

Generally chokes and capacitors may be used as effective radio noise suppressors. Such noise suppressors are effective where relatively high noise frequencies occur, but are relatively ineffective at frequencies less than 1 m Hertz, i.e., in the range of medium and long wave radiation. Even complex filter circuits which are applied to suppress radio noise resulting from mechanical switching devices are relatively ineffective to suppress radio noise resulting from ac phase control by means of thyristors, and the bulk and cost of such filter circuits tend to be excessive.

The prime object of the present invention is to provide means applicable in conjunction with a-c phase control for avoiding radio noise, and more particularly to control the rate of rise of the a-c phase controlled load current in such a way as to keep the high frequency components thereof at a permissible level, and to achieve these ends with a minimum of bulk, a minimum of cost and a minimum of parts.

SUMMARY OF INVENTION An a-c phase control circuitry embodying this invention includes an a-c power source, a load supplied from said power source, a power transistor controlling the flow of current from said power source to said load, means for effecting rapid ON- OFF switching of said power transistor to conductively connect said power source for a controlled fraction of each cycle of said power source to said load, and manually operable means for varying said controlled fraction of each cycle of said power source during which said power source is conductively connected to said load by the intermediary of said power transistor.

BRIEF DESCRIPTION OF DRAWINGS FIG. I is a diagrammatic representation of a dimmer circuit embodying the present invention;

FIG. 2a shows diagrammatically a current trace of a current occurring in a prior art a-c phase control circuit wherein a-c phase control is effected by a thyristor;

FIG. 2b shows diagrammatically a current trace of a current occurring in the circuit of FIG. 1 wherein a-c phase control is efiected by a transistor;

FIG. 3 is a complete circuit diagram of an a-c phase control circuit embodying the present invention;

FIG. 4 is a complete circuit diagram of a modification of the circuit of FIG. 3 particularly intended to form a monolithic integrated circuit of the LS! type; and

FIGS. 5a-5c are voltage and current traces referring to the circuitry of FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION In FIG. 1 reference characters a and b have been applied to indicate a pair of conductors or leads. An a-c voltage prevails between conductors or leads a and b. Reference numeral 5 has been applied to indicate a load, and more particularly an incandescent electric lamp. The circuit of load 5 includes disconnect switch 1 and full wave rectifier bridge 2. The latter is provided with four power diodes 2a,2b,2c,2d. Power transistor 3 conductively interconnects two of the terminals of full wave rectifier bridge 2, i.e., its d-c terminals. Reference character 6 has been applied to generally indicate a control circuit for transistor 3. Control circuit 6 can be adjusted by adjusting means such as knob or handle 7. By adjusting knob or handle 7 the fraction of half cycles during which power transistor 3 carries current from lead a to b may be varied. Control circuit 6 is connected by leads C,,C,,C and C to bridge circuit 2, the ends of leads C C, and C. remote from control circuit 6 being conductively connected to terminals of bridge circuit 2 and the end of lead C remote from control circuit 6 being conductively connected to the base of power transistor 3. Conu'ol circuit 6 may take various forms as long as it performs the function of a-c phase control of power transistor 6, i.e., controls the phase angle at which power transistor 6 is turned on periodically.

When lead a is positive a current may flow from lead a through closed disconnect switch I, diode 2a, power transistor 3, diode 2d, load 5 to lead b. When lead b is positive a current may flow from lead b, through load 5, diode 2c, power transistor 3, diode 2b, closed disconnect switch I to lead a. The rapid ON-OFF switching by transistor 3 conductively connects the a-c power supply a,b to load 5 for controlled fractions of each cycle, the particular fraction depending in each instance on the position of adjusting means 7.

FIGS. 20 and 2b show current plotted against time, orelectrical angles, respectively, for a total of 2 1r. In both instances current flow is initiated at the same electrical angle a. FIG. 2a referring to a thyristor a-c phase control circuit shows a very steep rise of the current I following triggering of the thyristor at the electrical angle a. The rate of rise of the current is )8, and B is virtually According to FIG. 2b referring to the circuit of FIG. 1 wherein phase-control is efiected by turning transistor lion and off, the rate of rise B, of the current is much less than in FIG. 2a, i.e. B, B or [3, 90. The current trace of FIG. 2a involves a wide band of frequencies, up to the highest frequencies. The current trace of FIG. 2b involves a much narrower band of frequencies, and these may be kept below a specified permissible band width N.

Referring now to FIG. 3, a-c lead a is conductively connected to a-c lead b by the intermediary of a full wave rectifier bridge including diodes 2a',2b',2c'2d' and power transistor 3'. Power transistor 3 includes emitter 3a, base 3b and collector 3c. beads C, and C are connected to the two do terminals of the above referred-to bridge circuits and the emitter 3a and the collector 3c of transistor 3' are conductively connected to the same terminals, a resistor 17' being included in the collector circuit of power transistor 3. A resistor 8' is arranged in lead C and leads C, and C, are conductively interconnected by Zener diode 9' limiting the (1-0 voltage prevailing across leads C,',C substantially at a predetermined level. Capacitor 10 and regulating resistor 1 l are connected in series and shunted across Zener diode 9'. Reference character 12 has been applied to generally indicate a unijunction transistor or silicon double base diode. The emitter of the latter is conductively connected to a point between capacitor 10 and regulating resistor 11'. One base of unijunction transistor 12' is conductively connected by lead C to one terminal of regulating resistor 11, and the other base of unijunction transistor 12 is conductively connected by lead C, to the gate of auxiliary thyristor 13. The latter interconnects the d-c terminals of bridge circuit 2a',2b',2c',2d',3', and resistor 14 is included in the anode circuit of auxiliary thyristor 13'. Resistor 16 connects the anode of auxiliary thyristor l3 and the base of power transistor 3'. Reference character 15' has been applied to indicate a capacitor of which one terminal is conductively connected to the base 3b of power transistor 3 and the other terminal is conductively connected to the emitter 3a of power transistor 3'.

The circuitry of FIG. 3 operates as follows:

When disconnect switch 1' is closed a d-c voltage appears across the d-c terminals of full wave rectifier bridge 2a',2b ',2c,2d,3. This d-c voltage prevails across leads C and C and is suitably reduced and stabilized by the operation of Zener diode 9 and the voltage drop occurring across resistor 8. The voltage across Zener diode 9 may be in the order of 10 volts and relatively constant, except around the points of time when the supply voltage across leads a',b drops to zero. Capacitor 10' is charged during each consecutive half cycle, the rate of charge of capacitor 10' depending upon the adjustment of regulating resistor 11'. When the voltage across capacitor 10' reaches a predetermined level unijunction transistor 12'. is turned on and capacitor 10' is discharged through a circuit including conductor C and the gate of auxiliary thyristor 13'. Thus auxiliary thyristor 13' is triggered. Triggering of auxiliary thyristor 13' establishes a direct current path from the positive to the negative terminal of full wave bridge 2a,2b',2c',2d',3' through auxiliary thyristor l3 and resistor 14 which are connected in series.

Initially the voltage across capacitor 15' is zero and, therefore, the voltage across emitter 3a and base 3b of power transistor 3' is also zero. Hence the load current I, from lead a through power transistor 3 and load 5' is also zero. Upon triggering of auxiliary thyristor 13 a voltage prevails across capacitor 15 and capacitor 15 is charged through resistor 16'. As a result the voltage between emitter 3a and base 3b of power transistor 3 increases and the load current I through load 5' increases likewise. The rate of rise of the load current I through power transistor 3' depends upon the magnitude of resistor 16.

During the time power transistor 3' is turned on the required base current flows for the remaining portion of onehalf cycle through a circuit which includes auxiliary thyristor 13'. In that period of time the triggering voltage for auxiliary thyristor 13' has collapsed.

Because the a-c phase control of load 5' is effected by means of a fullwave diode bridge, the operation of the circuitry of FIG. 3 is the same during each positive half cycle and during each negative half cycle.

Resistor 14 is dimensioned in such a way that the holding current of auxiliary thyristor I3 is reached shortly after the time of voltage zero. Resistor 17 is required for the proper control of the base current of power transistor 3'.

In FIG. 4 the parts corresponding to parts in FIG. 3 have been indicated by the same reference characters to which two primes rather than one single prime as in FIG. 3 have been added. Hence FIG. 4 is substantially self-explanatory, and requires additional description only to the extent that the circuitry shown therein differs from that shown in FIG. 3.

FIG. 4 differs from FIG. 3 only inasmuch as the power transistor 3' of the latter has been substituted in the former by a cascade of two transistors, i.e., auxiliary transistor 18 and power transistor 3". Auxiliary transistor 18" operates as an amplifier. Assuming that there is the same time constant in the circuitry of FIG. 3 and that of FIG. 4, the provision of auxiliary transistor 18" makes it possible to greatly increase the magnitude of resistor 16" relative to that of resistor 16' and to greatly reduce the magnitude of capacitor 16' relative to that of capacitor 16'. This is of considerable importance when it is intended to achieve an a-c phase control embodying this invention by integrated monolithic circuitry, for it is extremely difiicult, or impossible, to include in monolithic integrated circuits capacitors having a relatively high capacitance. I

Referring now to FIGS. 511,51: and 5c, the first mentioned figure shows the voltage U(3a,3c) across theemitter 3a and the collector 3c of power transistor 3 plotted against time. This voltage rises sinusoidally and collapses relatively rapidly when power transistor 3' is turned on. FIG. 5b shows the voltage U(15) prevailing across capacitor 15 plotted against time t andFIG. 5c shows the load current I, flowing through power transistor 3' and through load 5' plotted against time.

As indicated above, the kind of a-c phase control which has been disclosed above is not only applicable to dimmer circuits,

but also to many other a-c phase control circuits including, for

instance, speed controls of electric motors.

It will be apparent from the above that unijunction transistor 12' and capacitor 10 operate as a free running oscillation generator under the control of variable resistor 11'. This free running oscillation generator is energized by the voltage prevailing across the d-c terminals of fullwave rectifier bridge 2a',2b,2c',2d',3' and triggers auxiliary thyristor 13. The latter, in turn, controls the flow of current I through power thyristor 3' and load 5 I claim as my invention.

1. An a-c phase control circuitry with reduced radio noise including in combination a. an a-c power source;

b. a load supplied from said power source;

c. a full wave diode rectified bridge having a-c terminals conductively connected to said power source and d-c terminals conductively connected to a power transistor;

a free running oscillation generator energized by the d-c voltage prevailing between said d-c terminals of said rectifier bridge and controlling rapid ON-OFF switching of said power transistor;

. an auxiliary transistor cascade connected with said power transistor, said cascade connected power transistor and said auxiliary transistor being shunted by a capacitor;

f. said capacitor and other constituent circuit elements of said circuitry being formed by a monolithic integrated circuit; and

g. manually operable means for varying the fraction of each cycle of said power source during which said power source is conductively connected to said load.

2. An a-c phase control circuitry with reduced radio noise including in combination a. an a-c power source;

b. a load supplied from said source;

0. a full wave diode rectifier bridge having a-c terminals conductively connected to said power source and d-c terminals conductively connected to a power transistor;

. a free running oscillation generator energized by the d-c voltage prevailing between said d-c terminals of said rectifier bridge and controlling rapid ON-OF F switching of said power transistor, said free running oscillation generator being formed by a capacitor and a unijunction transistor, said unijunction transistor controlling the gate circuit of an auxiliary thyristor to trigger said auxiliary thyristor;

e. a capacitor shunted across two electrodes of said power transistor and charged bya voltage depending upon the flow of current through said auxiliary thyristor; and

f. manually operable means for varying the fraction of each cycle of said power source during which said power source is conductively connected to said load.

3. An a-c phase control circuitry as specified in claim 2 including lead means conductively connecting said d-c terminals of said rectifier bridge, a serially connected resistor and Zener diode providing reduced and stabilized operating voltage for said free running oscillation generator being included in said lead means.

4. An a-c phase control light dimmer circuitry with reduced radio noise including in combination a. an a-c power source;

b. an electric lamp supplied from said power source;

c. a power transistor controlling the flow of current from said power source to said electric lamp;

d. means for effecting rapid ON-OFF switching of said power transistor including an auxiliary transistor and a capacitor, said auxiliary transistor and said power transistor being cascade connected and said auxiliary transistor and said power transistor being shunted by a capacitor, and said capacitor and other constituent elements of said dimmer circuitry being formed by a monolithic integrated circuit; and

e. manually operable means for varying a controlled fraction of each cycle of said power source during which said power source is conductively connected to said electric lamp.

t I I t I 5 

1. An a-c phase control circuitry with reduced radio noise including in combination a. an a-c power source; b. a load supplied from said power source; c. a full wave diode rectified bridge having a-c terminals conductively connected to said power source and d-c terminals conductively connected to a power transistor; d. a free running oscillation generator energized by the d-c voltage prevailing between said d-c terminals of said rectifier bridge and controlling rapid ON-OFF switching of said power transistor; e. an auxiliary transistor cascade connected with said power transistor, said cascade connected power transistor and said auxiliaRy transistor being shunted by a capacitor; f. said capacitor and other constituent circuit elements of said circuitry being formed by a monolithic integrated circuit; and g. manually operable means for varying the fraction of each cycle of said power source during which said power source is conductively connected to said load.
 2. An a-c phase control circuitry with reduced radio noise including in combination a. an a-c power source; b. a load supplied from said source; c. a full wave diode rectifier bridge having a-c terminals conductively connected to said power source and d-c terminals conductively connected to a power transistor; d. a free running oscillation generator energized by the d-c voltage prevailing between said d-c terminals of said rectifier bridge and controlling rapid ON-OFF switching of said power transistor, said free running oscillation generator being formed by a capacitor and a unijunction transistor, said unijunction transistor controlling the gate circuit of an auxiliary thyristor to trigger said auxiliary thyristor; e. a capacitor shunted across two electrodes of said power transistor and charged by a voltage depending upon the flow of current through said auxiliary thyristor; and f. manually operable means for varying the fraction of each cycle of said power source during which said power source is conductively connected to said load.
 3. An a-c phase control circuitry as specified in claim 2 including lead means conductively connecting said d-c terminals of said rectifier bridge, a serially connected resistor and Zener diode providing reduced and stabilized operating voltage for said free running oscillation generator being included in said lead means.
 4. An a-c phase control light dimmer circuitry with reduced radio noise including in combination a. an a-c power source; b. an electric lamp supplied from said power source; c. a power transistor controlling the flow of current from said power source to said electric lamp; d. means for effecting rapid ON-OFF switching of said power transistor including an auxiliary transistor and a capacitor, said auxiliary transistor and said power transistor being cascade connected and said auxiliary transistor and said power transistor being shunted by a capacitor, and said capacitor and other constituent elements of said dimmer circuitry being formed by a monolithic integrated circuit; and e. manually operable means for varying a controlled fraction of each cycle of said power source during which said power source is conductively connected to said electric lamp. 